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J Am Chem Soc 2025[Nov]; ? (?): ? PMID41319233show ga
Supported chromium catalysts find broad application due to metal's ability to assume a range of oxidation states and structures. However, the behavior of chromium species on reducible metal oxides under reactive conditions remains poorly understood, despite the potential for unique reactivity through metal-support interactions. Recent studies on CeO(2)-supported Cr(n+) single atoms for selective NH(3) oxidation to N(2)O underscore this potential, attaining high performance through cocatalytic action with CeO(2), but also suffering from deactivation through agglomeration of isolated sites into Cr(2)O(3). To address this challenge, we investigate how distinct CeO(2)-supported chromium species evolve under varying reactive environments. We show that under oxidative conditions redispersion of Cr(2)O(3) occurs, serving as a catalyst regeneration strategy and enabling the recovery of both structure and performance. Combining advanced microscopy, in situ Raman, UV-vis, electron paramagnetic resonance and X-ray absorption spectroscopies, we follow the transformation from crystalline Cr(2)O(3) nanoparticles to isolated chromium species. The redispersion is proposed to proceed via particle amorphization and oxidation of Cr(3+) to mobile Cr(6+), which diffuse over the support and stabilize as chromate species or as Cr(5+) upon reduction by oxygen vacancies. A slower redispersion rate is observed over less reducible ZrO(2) and TiO(2), with minimal changes on gamma-Al(2)O(3) and Nb(2)O(5), highlighting support reducibility as the driver of the process and reflecting the potential for generating Cr(6+) sites from nontoxic Cr(2)O(3) by controlling support properties. These results demonstrate how redox-active supports facilitate reversible changes to metal nanostructure, offering a promising strategy for regenerating catalysts and tuning metal speciation through rational support design for various applications.
J+Am+Chem+Soc 2025 ; ? (?): ?
